1. Field of the Invention
This invention relates generally to a method for automated processing of medical images, and more particularly to an automated method for the detection and classification of abnormal regions, particularly microcalcifications in digital mammographic images. The present invention further relates to the subject matter described in commonly owned U.S. Pat. Nos. 5,289,374, 5,133,020 and 4,907,156 and pending U.S. patent applications Ser. Nos. 07/843,721, now U.S. Pat. No. 5,343,390, and 07/915,631, now U.S. Pat. No. 5,537,485.
2. Discussion of the Background
Detection and diagnosis of abnormal anatomical regions in radiographs, such as cancerous lung nodules in chest radiographs and microcalcifications in women's breast radiographs, so called mammograms, are among the most important and difficult task's performed by radiologists.
Recent studies have concluded that the prognosis for patients with lung cancer is improved by early radiographic detection. In one study on lung cancer detection, it was found that, in retrospect, 90% of subsequently diagnosed peripheral lung carcinomas were visible on earlier radiographs. The observer error which caused these lesions to be missed may be due to the camouflaging effect of the surrounding anatomical background on the nodule of interest, or to the subjective and varying decision criteria used by radiologists. Underreading of a radiograph may be due to a lack of clinical data, lack of experience, a premature discontinuation of the film reading because of a definite finding, focusing of attention on another abnormality by virtue of a specific clinical question, failure to review previous films, distractions, and "illusory visual experiences".
Similarly, early diagnosis and treatment of breast cancer, a leading cause of death in women, significantly improves the chances of survival.
X-ray mammography is the only diagnostic procedure with a proven capability for detecting early-stage, clinically occult breast cancers. Between 30 and 50% of breast carcinomas detected radiographically demonstrate microcalcifications on mammograms, and between 60 and 80% of breast carcinomas reveal microcalcifications upon microscopic examination. Therefore any increase in the detection of microcalcifications by mammography will lead to further improvements in its efficacy in the detection of early breast cancer. The American Cancer Society has recommended the use of mammography for screening of asymptomatic women over the age of 40 with annual examinations after the age 50. For this reason, mammography may eventually constitute one of the highest volume X-ray procedures routinely interpreted by radiologists.
A computer scheme that alerts the radiologist to the location of highly suspect lung nodules or breast microcalcifications should allow the number of false-negative diagnoses to be reduced. This could lead to earlier detection of primary lung and breast cancers and a better prognosis for the patient. As more digital radiographic imaging systems are developed, computer-aided searches become feasible. Successful detection schemes could eventually be hardware implemented for on-line screening of all chest radiographs and mammograms, prior to viewing by a physician. Thus, chest radiographs ordered for medical reasons other than suspected lung cancer would also undergo careful screening for nodules.
Several investigators have attempted to analyze mammographic abnormalities with digital computers. However, the known studies failed to achieve an accuracy acceptable for clinical practice. This failure can be attributed primarily to a large overlap in the features of benign and malignant lesions as they appear on mammograms.
The currently accepted standard of clinical care is such that biopsies are performed on 5 to 10 women for each cancer removed. Only with this high biopsy rate is there reasonable assurance that most mammographically detectable early carcinomas will be resected. Given the large amount of overlap between the characterization of abnormalities may eventually have a greater impact in clinical care. Microcalcifications represent an ideal target for automated detection, because subtle microcalcifications are often the first and sometimes the only radiographic findings in early, curable, breast cancers, yet individual microcalcifications in a suspicious cluster (i.e., one requiring biopsy) have a fairly limited range of radiographic appearances.
The high spatial-frequency content and the small size of microcalcifications require that digital mammographic systems provide high spatial resolution and high contrast sensitivity. Digital mammographic systems that may satisfy these requirements are still under development. Digital radiographic systems with moderately high spatial resolution are made possible by fluorescent image plate/laser readout technology. Currently, digital mammograms with high resolution can be obtained by digitizing screen-film images with a drum scanner or other scanning system. The increasing practicability of digital mammography further underlines the potential ability of a computer-aided system for analysis of mammograms.
The inventors and others at the Radiology Department at the University of Chicago have been developing a computerized scheme for the detection of clustered microcalcifications in mammograms with the goal of assisting radiologists' interpretation accuracy. (See H. P. Chan et al., "Image feature analysis and computer-aided diagnosis in digital radiography. 1. Automated detection of microcalcifications in mammography", Med. Phys. 14, 538-548 (1987); H. P. Chan et al., "Computer-aided detection of microcalcifications in mammograms: Methodology and preliminary clinical study", Invest. Radiol. 23, 664-671 (1988); H. P. Chan et al., "Improvement in radiologists' detection of clustered microcalcifications on mammograms: The potential of computer-aided diagnosis", Invest. Radiol. 25, 1102-1110 (1990); R. M. Nishikawa et al., "Computer-aided detection and diagnosis of masses and clustered microcalcifications from digital mammograms", Proc. SPIE 1905, 422-432 (1993); and R. M. Nishikawa et al., "Computer-aided detection of clustered microcalcifications: An improved method for grouping detected signals", Med. Phys. 20, 1661-1666 (1993).)
The computer outputs from this scheme, which involves quantitative analysis of digitized mammograms, indicate possible locations of clustered microcalcifications. These locations can be marked by arrows superimposed on mammograms displayed on the monitor of a workstation. (See U.S. Pat. No. 4,907,156.) If the computer output is presented to radiologists as a "second opinion" (see K. Dei et al., "Digital radiography: A useful clinical tool for computer-aided diagnosis by quantitative analysis of radiographic images", Acta Radiol. 34, 426-439 (1993); and M. L. Giger, "Computer-aided diagnosis", RSNA Categorical Course in Physics, 283-298 (1993)), it is expected that the accuracy in detecting clustered microcalcifications in mammograms would be improved by reducing false-negative detection rate. The prior computer-aided diagnosis (CAD) scheme has a sensitivity of approximately 85% with 2.5 false-positive clusters per mammogram. Since the sensitivity is at a relatively high level, a reduction of false-positive detection rate is desired before beginning clinical testing. The prior scheme uses the first moment of the power spectrum and the distribution of microcalcification signals to eliminate false-positive microcalcification signals. To reduce further the false-positive rate, new techniques, including application of an artificial neural network (see pending U.S. patent applications Ser. Nos. 08/053,345, now U.S. Pat. No. 5,463,548 and 08/060,531, now U.S. Pat. No. 5,491,627) and an area-thickness analysis (see Y. Jiang et al., "Method of extracting microcalcifications' signal area and signal thickness form digital mammograms", Proc SPIE 1778, 28-36 (1992)) have been investigated and have been shown to be effective.
However, further improvement in microcalcification detection sensitivity and diagnostic accuracy is desired.